1. Field of the Invention
The invention relates to Ultra Wide Band (UWB) communications, and in particular, to a method and apparatus for frequency synthesizing.
2. Description of the Related Art
IEEE 802.15.3a is a standard for wide bandwidth and short range data communications. Orthogonal Frequency Division Multiplexing (OFDM) Ultra Wide Band (UWB) communication system is based upon the IEEE standard. UWB is defined as any radio technology having a spectrum that occupies a bandwidth greater than 20 percent of the center frequency, or bandwidth of at least 500 MHz. Modern UWB systems use modulation techniques such as OFDM to occupy these extremely wide bandwidths, thereby enabling high bit rate up to 480 Mbps within a short distance.
A UWB spectrum is shown in FIG. 1. The UWB spectrum from 3.1 to 10.6 GHz is divided into 14 bands of 528 MHz each, wherein 14 carrier frequencies are correspondingly associated. These bands are further grouped into five band groups, each having two or three consecutive bands. Conventionally, generation of the carrier frequencies involves usage of 14 independent oscillation sources. Various enhancements have been proposed to generate the carrier frequencies such as using fewer oscillation sources. For example, in Mishra's US patent publication US2007/0066262, one frequency synthesizer uses a single oscillator and a plurality of dividers to generate all 14 UWB bands. The bands are generated in two phases. In phase one, bands 2, 5, 8, 11 and 14 are first generated. In phase two, the remaining bands are generated by frequency translation of the bands 2, 5, 8, 11 and 14. In this architecture, only one oscillator source is required, however, the two phase process is deemed ineffective since generation of the bands 1, 3, 4, 6, 7, 9, 10, 12 and 13 require additional circuits to perform frequency translation, wherein multiple stages of single side band (SSB) mixers are deployed. One of the disadvantages is harmonic distortion, which worsen along multiple stages as I/Q mismatch increases. Hence, Mishra's two-phase implementation is cost ineffective while performance is degraded. Therefore, an enhancement is desirable.